X-ray Fluorescence Analysis of Zinc and Zinc-iron Alloy Coated Steel Sheet

Wavelength-dispersive XRF determines zinc coating weight from the intensity of a zinc fluorescence line. Conventional measurements commonly use Zn-Kα, but its intensity approaches saturation for thick coatings, limiting the practical measurement range to about 200 g/m² under standard optical conditions. Using the more penetrating Zn-Kβ1 line together with an increased take-off angle and a specialized primary beam filter extends accurate measurement to zinc coating weights as high as 450 g/m².

Galvannealed steel is produced by heating zinc-coated steel so that iron diffuses from the substrate into the zinc coating and forms zinc–iron alloy phases. Too little iron indicates insufficient alloying and can leave a soft surface layer that tends to adhere to forming dies and cause flaking. Excessive iron can produce a thicker, hard and brittle iron-rich layer that is susceptible to powdering during press forming. Controlling iron concentration is therefore essential to maintaining the desired forming properties and coating quality.

A thin-film fundamental parameter method uses X-ray intensities obtained from different analyzing depths within the coating. Zn-Kβ1 measured at a 40° take-off angle and Zn-Kα measured at a 20° take-off angle provide sufficiently different penetration depths while still probing deeply enough into the zinc–iron alloy layer. The two measurements can then be used in simultaneous calculations to determine both total coating weight and iron concentration.

Although Zn-Kα provides information from relatively deep within the coating, Zn-Lα has an analyzing depth of only about 10 g/m² under the evaluated conditions. It therefore represents primarily the surface region rather than the entire zinc–iron alloy coating. Because iron concentration varies through the coating thickness, this shallow measurement does not adequately characterize the whole layer. A Zn-Kα/Zn-Lα thin-film FP method produced an iron-concentration accuracy of only 1.59 mass% over an 8–13 mass% range.

When X-ray lines are measured at different take-off angles, small differences in sample height can change measured intensities and consequently introduce errors into calculated coating weight and composition. Normalizing each zinc intensity to an Fe-Kα intensity measured at the same take-off angle substantially reduces this effect. With a 0.25 mm height change, the iron-concentration error decreased from −0.69 mass% using direct intensities to 0.11 mass% using intensity ratios.

For galvanized steel with coating weights between 150 and 450 g/m², the reported coating-weight accuracy was 4.57 g/m². A sample with an average coating weight of 402.2 g/m² gave a coefficient of variation of 0.26%. For galvannealed steel, thin-film FP analysis achieved accuracies of 0.71 g/m² for coating weight and 0.27 mass% for iron concentration. Repeatability coefficients of variation were 0.07% for coating weight and 0.15% for iron concentration.

Conventional batch analysis requires selectively dissolving the plating layer, determining coating weight from the resulting weight change, and chemically analyzing the solution for iron. The procedure requires technical expertise and is relatively time-consuming. XRF eliminates coating dissolution, provides rapid nondestructive measurements, and can simultaneously determine coating weight and iron concentration in galvannealed steel, making it particularly useful when large numbers of quality-control samples must be analyzed.

Advanced high-strength steels can contain substantial amounts of alloying elements such as Ni, Mn, Si, and Nb. During galvannealing, these elements may diffuse into the coating along with iron. Because thin-film FP calculations depend on assumptions about substrate and coating composition, a single calibration may not provide sufficient accuracy across steel grades with significantly different compositions. Dividing the FP calibration according to steel type can improve analytical accuracy.